Diverse recreational bodies of water are prone to be affected by bacteria and microalgae proliferation. In cases where the quality of water is of great importance, like swimming pools, high quality water suitable for swimming is achieved by adding large amounts of chemical agents. In swimming pools, for example, chemical agents are added to the water in the pool in order to maintain at least a 1.5 ppm permanent concentration of chlorine agents in the water. This concentration is required due to the strict swimming pool regulations regarding the water's bacteriological and physicochemical properties, and it is achieved by maintaining a permanent Oxidation Reduction Potential (ORP) on the water of at least 650 mV on a continuous basis.
Many countries throughout the world have regulations regarding recreational waters and there are generally two types of regulations regarding recreational use of such water bodies. The first type of regulation is directed to swimming pools, which essentially requires the maintenance of a high permanent chlorine buffer in order to avoid the contamination of the water when new bathers enter the swimming pool. The chlorine buffer neutralizes contaminants and kills microorganisms brought to the swimming pool water by bathers, thus maintaining a high water quality suitable for recreational purposes.
The second type of regulation applies to lakes and the sea, and is referred to as the criteria for bathing with full body contact for recreational waters. This regulation is based on the diluting power of water. When new bathers enter a body of water, the contaminants are diluted in such a way that the contaminants do not attain a concentration in the body of water that causes significant effects. Therefore, in a large water body such as a lake or sea, a disinfectant buffer is not needed due to the high diluting power of the large water volume.
There is a global trend towards more sustainable and environmentally conscious systems in all aspects of life that can coexist with the environment leaving the smallest footprint behind. Nevertheless, when it comes to the treatment of recreational bodies of water, there have not been substantial advances towards environmentally responsible methods and systems. Usual treatments regarding the proliferation of bacteria and microalgae in swimming pools require high amounts of chemicals and energy, which does not fulfill today's sustainable necessities.
In order to comply with the first regulation, a chlorine buffer in swimming pools must be maintained on a continuous basis to neutralize the contaminants brought into the pool by new bathers. Maintaining a permanent chlorine buffer in the traditional treatment of swimming pools is related to maintaining an ORP level of at least 650 mV on a continual basis. Once demand for chlorine disinfecting has been satisfied, the free chlorine concentration remaining in the water, i.e. the residual chlorine, acts as a buffer in order to provide disinfection when new organic matter or microorganisms enter the body of water, for example, when new bathers enter into the water. The amount of chemicals used to permanently maintain an ORP level of at least 650 mV is very high, significantly increases the operational costs of the pool, involves the use of chemicals in amounts that are not environmentally friendly and can generate undesirable disinfection by-products (DBP's), such as chloramines.
Additionally, a typically configured swimming pool requires the filtration of its entire volume of water usually from 1 to 6 times per day. This is achieved by using a traditionally configured centralized filtration unit. Thus, the filtration system of traditional swimming pools consumes large amounts of energy, and also imposes high demands in terms of installation, operation and maintenance costs.
In sum, traditional swimming pools depend on large amounts of chemicals to maintain a chlorine buffer in order to neutralize the contamination entering the water body and a centralized filtration system that filters the whole volume of water usually from 1 to 6 times per day. Thus, traditional swimming pool technology has high operational and maintenance costs due to the high disinfectant concentrations that have to be continuously maintained and the required centralized filtration system. There is a great need for a sustainable, low energy method and system for treating and maintaining large water volumes for recreational purposes using a small amount of chemicals.
Swimming Pools
The consumption of nutrients in water by aerobic microorganisms results in a high demand of oxygen. This in turn, lowers the levels of dissolved oxygen in the water, and thus, allows the development of anaerobic microorganisms. Further development of anaerobic microorganisms produces accumulation of organic compounds. This chain of events produces an accumulation of nutrients in water, which can serve as a culture broth for particular microorganisms. Among the microorganisms developed in these conditions, bacteria and microalgae are the most relevant in recreational water bodies.
The turbidity in recreational bodies of water is produced mainly by microalgae present in the water. These microorganisms grow in water with a determined concentration of nutrients. Depending on the availability of a light source and the concentration of nutrients, algal blooms can occur in a process called eutrophication in which the algae populates the whole volume of water turning the whole body of water into a huge source of biomass and increases the turbidity of the water. Different regulations indicate that values of up to 50 nephelometric turbidity units (NTU) would be safe for human health. In British Columbia, for example, water turbidity determined by the Environmental Protection Division of the Ministry of Environment for recreational bodies of water, establishes an upper limit of 50 NTU (Section 2(e) of the Environment Management Act, 1981 of British Columbia, Canada), whereas the South Australian government considers an upper value of 25 NTU. When dissolved nutrients required for the maintenance of microalgae are consumed, the microalgae die and settle to the bottom of the body of water. The available organic matter in this settled layer at the bottom of the body of water can again serve as basis for the development of anaerobic microorganisms in the water, which in general pose health threats for humans. In artificial bodies of water, the quality of water deteriorates progressively as a consequence of microalgae and bacteria growth. In typical methods or processes for water treatment of swimming pools with high density of bathers, large amounts of disinfectant agents, such as chlorine, together with the filtration of the entire volume of water are used to control the proliferation of bacteria and microalgae. For example, if chlorine is used, it will react with organic matter as well as with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions. The chlorine consumed in these reactions is defined as the chlorine demand. To meet the chlorine demand, permanent ORP levels of at least 650 mV have to be maintained in the water.
Reaction of chlorine with the organic compounds present in the water can form several toxic by-products or disinfection by-products (DBP). For example, the reaction of chlorine with ammonia can produce chloramines as undesired by-products. Further reaction of chlorine or chloramines with organic matter can produce trihalomethanes, which have been implicated as potential carcinogens. Also, depending on the disinfection method, new DBPs have been identified, such as iodinated trihalomethanes, haloacetonitriles, halonitromethanes, haloacetaldehydes, and nitrosamines. Furthermore, exposure of bathers to chlorine and organic matter has been suggested as contributing to respiratory problems, including asthma, and several other health problems.
The use of chemical agents also poses an environmental problem related to the accumulation and the disposal of these chemicals and DBPs in the environment. Therefore, a reduction in the use of such chemicals and the resultant DBPs would be beneficial.
In addition to the high cost and health and environmental issues associated with chemical treatment, conventional filtration systems have high capital costs and energy consumption. Traditional treatments of standard sized swimming pools require the filtration of the whole volume of water, usually from 1 to 6 times a day using a traditionally configured centralized filtration unit. This type of system imposes a high demand on energy supply, as well as elevated capital costs associated with the filtration system such as piping, pumps, filters, and facilities, among others.